john wrote:

I always assumed that it was done for convenience. I have 3 hot wires
(A, B & C(wild)), I can run all my 120 stuff off A to neutral or B to
neutral. I can run all of my 240 single phase stuff off A-B and I can
run all of my 240 3-phase stuff off A-B-C.



Anything that uses 220 only, ( 220 with two conductors and a ground) can
be run off of any two legs. Only if the thing uses 120 and 240 volts
and has a neutral wire going to it do you need to use the centertapped
phases.


If I had Wye or corner grounded delta service then I would have to
give up one of those conditions or purchase another transformer.

Nope

One thing that is common in warehouses here is 208 Wye service. That
gives 3, 120 Volt legs. Obviously, I can run a 240 3-phase motor on
that service but it will only run at 208/240)^2 = 75% power. Worse
yet, many electronic items just simply will not run.

Alternatively, I can get 240 Wye service and run my 240 stuff at full
power. As you mentioned, you can always run 240 single phase stuff
across any two legs. (I can also use my 208V light bulbs from the
previous example) But how do I get my 110V toaster oven to work in
that arrangement?

Same question for a corner grounded delta arrangement?

By the way: center tapped delta service is very common here in the
Chicago area. Wye service is strictly used in office buildings and
warehouses.



It is good for the electrical company when there is a mixed use of
single and light three phase users. They save a transformer when three
phase is required.


As a side note: I have a suspicion that Commonwealth Edison balances
the overall service in an industrial park by locally grounding the
center of alternate coils. IOW: that wire that comes into my building
and that I call the wild leg, is not the same as my neighbor's wild
leg.

All utility companies alternate on the hookup of the primary to the
three phase high voltage lines, the ones on the extreme top of the pole
with the big insulators.

you neighbor may be on a different pole transformer. IF he is then his
phasing is different than yours.

Yea, I figured as much.

We had an electrical storm here several years ago. Half the light
bulbs in the building were getting dim while the other half were
getting unusually bright. Then some of the bright bulbs began to
blow. I figured that we must have lost the center ground and that the
voltage was being split somewhere in the middle of the center tapped
leg. No doubt based on the relative load on either side.

I ran round the building frantically shutting everything off as light
bulbs popped all around me. I then called the electric company and
explained the situation. I must hand it to them, they arrived in truly
record time!

But I noticed that my neighbors lights were also bright in one window
and dim in the next. I was just sort of pondering the fun that it
would be to ground out one side of the transformer and then ground out
the other side.

Com Ed arrived before I worked up enough nerve to put theory to
practice.